Self interference is a problem for wireless and other communications devices which attempt to send, e.g., transmit, and receive at the same time using electrical or wireless signals. While different frequency bands may be used for sending, e.g., uplink, and receiving, e.g., downlink, some of the signal being transmitted may be received by a receiver of the device, interfering with the receipt of signals being received from one or more other devices, Interference from the transmitter to the receiver of a device particularly in the case of a shared transmit and receive antenna or cable, or a transmit and receive antenna in close proximity between the transmitter and receiver which is often the case for antennas on mobile communications devices can create interference problems even when the transmit and receive frequency bands are different.
Attempts at canceling self interference by using one or more electronic circuits and filters implemented as electrical components operating in same frequency range as the signals being received and transmitted, e.g., radio frequency domain, have had limited success.
The generation of an interference cancelation signal using electrical components in the form of filters, etc., e.g., in the radio frequency domain, has several problems associated with it. For one thing the electrical circuit elements used to generate an interference cancelation signal may themselves radiate interference, particularly when dealing with signals in the RF frequency band, as wires and/or other components of a filter implemented as an electrical circuit operate as signal transmitters and receivers. Such additional self interference can be highly undesirable in a communications device which transmits and receives radio signals, e.g., using antennas, or electrical signals, e.g., using a electrical cable interface such as a coax cable, Ethernet cable or other non-optical cable. Another problem with the use of filters that operate in the RF band is that shielding within a small device to prevent the transmission of interference generated by such filters in the small device can be difficult to implement given space constraints.
Electrical filter circuits which may be used to generate an interference cancelation filter also have the disadvantage of being relatively bulky making it difficult to implement a large number of filter taps and/or separate delays in an electrical filter being used to generate an interference cancelation filter. For this reason attempts to generate an interference cancelation signal using electrical components operating in the RF frequency domain are often limited to using filters with very few taps and/or delays. Furthermore attempts to pack large numbers of RF circuits or filter taps in a small space can further complicate the problem of interference from one component leaking to another component via unintended radio frequency interaction between nearby components, e.g., with one component acting as an unintended RF transmitter and another component acting as an unintended RF receiver.
Power issues with splitting an electrical signal are also of concern as is thermal noise with electrical components that may be used to generate an interference cancelation signal using electrical components. If a weak interference signal is to be generated for one or more received signal components or frequencies the thermal noise of the electrical circuits may preclude the generation of a meaningful interference cancelation signal since the thermal noise of the electrical circuits used to generate the interference signal may, in some cases, exceed the expected interference signal to be canceled. Moreover, insertion losses in RF systems with couplers and/or microstrips can be high and should normally be impedance-matched carefully, keeping the resulting capacitance and inductances in mind, making the use of such components increasingly more challenging for higher RF frequencies.
Another problem with implementing filters as electrical circuits in the RF range is that it may be difficult to design or implement electrical circuits with the desired filter characteristics since frequency range of the filters may not be uniform in the desired frequency range that may be required to generate an appropriate interference cancelation signal.
While attempts to determine an appropriate interference cancelation signal in a digital RF domain may be attempted, to generate an accurate analog interference cancelation signal to be combined with a received signal may require digital to analog converters with a very large frequency range and resolution which can be costly and/or difficult to implement.
In view of the above discussion it should be appreciated that there is a need for improved methods and apparatus which can be used for self interference cancelation in which devices communicate using radio frequency signals. In particular it would be desirable if methods and/or apparatus could be developed which address, overcome or reduce one or more of the above discussed problems associated with generating interference cancelation signals using electrical circuits and/or filters operating in the radio frequency domain and/or require the conversion of a digital interference cancelation signal generated in the RF domain to an analog interference cancelation signal using a high resolution electrical digital to analog signal converter.